1. Field of the Invention
The present invention relates to a thermistor element which can detect a temperature ranging from room temperature to high temperature of about 1000.degree. C., i.e. so-called wide-range type thermistor element, and the thermistor element is particularly suitable for use in a temperature sensor for an automobile exhaust gas.
2. Description of the Related Art
A thermistor element for a temperature sensor is used in the measurement of a temperature ranging from moderate to high temperature (e.g. 400 to 1300.degree. C., etc.) such as temperature of an automobile exhaust gas, gas flame temperature of gas hot-water supply device, temperature of a heating oven, etc.
Characteristics of this kind of a thermistor element are indicated by the resistivity and resistivity temperature coefficient (temperature dependence of the resistivity). In order to cope with a practical resistivity range of a temperature detecting circuit constituting the temperature sensor, it is desired that the resistivity of the thermistor element is within a predetermined range. Therefore, perovskite materials are exclusively used as those having resistivity characteristics suitable for a wide-range type thermistor element.
As the thermistor element using perovskite material, for example, those described in Japanese Patent Kokai Publication Nos. Hei 6-325907 and Hei 7-201528 are suggested. These thermistor elements are produced by mixing oxides of Y, Sr, Cr, Fe, Ti, etc. in a predetermined composition proportion and calcining the mixture to form a perfect solid solution in order to realize a thermistor which can used in a wide temperature range.
The resistivity characteristics of the wide-range type thermistor element are indicated by the resistivity and resistivity temperature coefficient. In a normal temperature sensor, it is necessary that the resistivity of the thermistor element is from 50 to 300 k.OMEGA. within a working temperature range in view of the resistivity range of the temperature detecting circuit. In case of affording a heat history from room temperature to 1000.degree. C. to the thermistor element, the smaller a change between the resistivity after heat history and the initial resistivity, the better.
In the above Japanese Patent Publications, various thermistor elements of a perfect solid solution are suggested, but only data of the thermistor element resistivity at 300.degree. C. or more are disclosed. Therefore, the present inventors have examined the resistivity characteristics at about room temperature of various thermistor elements in the above Japanese Patent Publications.
As a result, regarding those having a resistivity stability in the heat history from room temperature to 1000.degree. C., the resistivity becomes higher in the temperature range from room temperature to 300.degree. C. Therefore, it is impossible to discriminate it from insulation and the temperature can not be detected. On the other hand, regarding those satisfying low resistivity of 50 to 300 k.OMEGA. the resistivity changes by 10% or more relative to the initial resistivity in the heat history. It has been found that the stability is poor.
There has never been obtained a thermistor element which can satisfy two resistivity characteristics which are contrary to each other, i.e. low resistivity characteristics within a range from room temperature to high temperature of 1000.degree. C. and resistivity stability in the heat history (so-called wide-range type thermistor element).
In the light of the above problems, an object of the present invention is to provide a thermistor element which has stable characteristics (i.e. small change in resistivity in the heat history from room temperature to 1000.degree. C.) and has a resistivity of 50 to 300 k.OMEGA. within the temperature range from room temperature to 1000.degree. C.